Field of the Invention
The present invention relates to a method and a device for regulating the temperature range of an NOx accumulator in an exhaust system for purifying an exhaust gas stream of an internal combustion engine. The invention can be used, in particular, for purifying an exhaust gas stream in a diesel or lean-burn engine for clearing away substances present in the exhaust gas, such as unburned hydrocarbons, carbon monoxide and nitric oxides.
Increasing environmental awareness and more stringent exhaust gas legislation reflecting that environmental awareness make it necessary to even further reduce exhaust gas components which are contained in the exhaust gas of an internal combustion engine and are classified as harmful. Normally, in present-day motor vehicles, a three-way catalytic converter is used, through the use of which carbon monoxide (CO), unburned hydrocarbons (HC) and nitrogen oxides (NOx) are converted into harmless constituents.
European Patent 0 298 240 B1 states that, in the known exhaust gas catalytic converter systems, the exhaust gas behavior of the internal combustion engine can be deduced by determining the residual oxygen content in the exhaust gas through the use of an oxygen probe. That is done by determining the temperature profile at the catalytic converter and by determining the heat quantity released during the catalytic reaction.
Pollutants occur in an internal combustion engine during operation and, above all, during the cold-starting phase. It is therefore necessary to provide catalytic converters in the exhaust tract of an internal combustion engine. That is known, for example, from European Patent 0 628 134 B1. It is known, furthermore, from European Patent 0 485 179 B1 to use absorbers for the storage of unburned hydrocarbons which occur during the cold-starting phase and are discharged again when the downstream catalytic converter is ready for operation.
Where stationary internal combustion engines are concerned, the Air Pollution Regulations apply in Germany. The reduction in nitric oxide emissions from the exhaust gas of internal combustion engines is often carried out through the use of urea in the case of stationary systems.
That is not possible in internal combustion engines for motor vehicles, specifically inter alia, because of the tank to be carried along for that purpose and because of its weight. Particularly where commercial vehicles are concerned, it is known to carry out exhaust gas recirculation through the use of recooled exhaust gases. It is also known to reduce NOx emissions by water injection. It is known, furthermore, to reduce NOx emissions by first storing them intermediately in an NOx accumulator or storage device, from which they are then reduced to nitrogen through the use of controlled reactions with unburned hydrocarbons. In other words, the oxygen is thereby extracted from the NOx constituents.
The normal operating temperature range, within which NOx accumulators of that type operate reliably, is approximately 150° C. to 500° C. at the present time. The aim is to achieve an increase in the upper temperature to, for example, 700° C. through the use of new coatings. Above a maximum temperature of 800° C. at the present time, NOx accumulators are damaged, and therefore such temperatures must be avoided under all circumstances. NOx accumulators of that type are disposed in the exhaust system, as a rule downstream of a first catalytic converter. The reaction taking place in the first catalytic converter, in particular that of hydrocarbons with oxygen, is exothermal, so that a heat flow is supplied to the exhaust gas while the latter is being purified in the catalytic converter. Since the NOx accumulator only reaches its minimum operating temperature at a temperature of approximately 150° C., it is desirable, above all with a view to the cold-starting behavior of an exhaust gas purification system of that type, to place the NOx accumulator as closely as possible downstream of the first catalytic converter. When the internal combustion engine is in the full-load mode, during the exothermal reaction taking place in the first catalytic converter, exhaust gas temperatures downstream of the catalytic converter are reached which may be in the region of or above 100° C. Under those full-load conditions, it is therefore desirable to place the NOx accumulator as far as possible downstream of the first catalytic converter, in order to ensure that, even under those full-load conditions, the temperature of the NOx accumulator does not rise above approximately 800° C. The two conditions conflict with one another.